1. Technical Field
The present invention relates generally to an apparatus for measuring the position of the mirror of a diffractive light modulator and performing positional compensation and a method of controlling the apparatus, and, more particularly, to an apparatus for measuring the position of the mirror of a diffractive light modulator and performing positional compensation, which measures the position of the mirror of a diffractive light modulator by measuring the capacitance of the mirror, the capacitance of a piezoelectric material layer or the intensity of output diffracted light and performs positional compensation, and a method of controlling the apparatus.
2. Description of the Related Art
With the development of microtechnology, Micro-Electro-Mechanical Systems (MEMS) devices and small-sized equipment, into which MEMS devices are assembled, are attracting attention.
A MEMS device is formed on a substrate, such as a silicon substrate or a glass substrate, in microstructure form, and is a device into which an actuator for outputting mechanical actuating force and a semiconductor Integrated Circuit (IC) for controlling the actuator are electrically or mechanically combined. The fundamental feature of such a MEMS device is that an actuator having a mechanical structure is assembled in part of a device. The actuator is electrically operated using Coulomb's force between electrodes.
FIGS. 1 and 2 illustrate the constructions of representative optical MEMS devices that utilize the reflection or diffraction of light and are applied to optical switches or light modulation devices.
The optical MEMS device 1 illustrated in FIG. 1 includes a substrate 2, a substrate side electrode 3 formed on the substrate 2, a cantilevered beam 6 configured to have an actuation side electrode 4 that is disposed opposite and parallel to the substrate side electrode 3, and a support 7 configured to support one end of the cantilevered beam 6. The beam 6 and the substrate side electrode 3 are electrically insulated from each other by a gap 8.
In the optical MEMS device 1, the beam 6 is displaced by electrostatic attractive force or electrostatic repulsive force generated between the beam 6 and the substrate side electrode 3 depending on electrical potential applied between the substrate side electrode 3 and the actuation side electrode 4. For example, as illustrated by the solid and dotted lines of FIG. 1, the beam 14 is displaced parallel to the substrate side electrode 3.
An optical MEMS device 11 illustrated in FIG. 2 includes a substrate 12, a substrate side electrode 13 formed on the substrate 12, and a beam 14 formed across the substrate side electrode 13 in bridge form. The beam 14 and the substrate side electrode 13 are electrically insulated from each other by a gap 10.
The beam 14 includes bridge members 15 configured to have a bridge shape and made of, for example, an SiN film, and an actuation side electrode 16 supported by the bridge members 15 to be opposite and parallel to the substrate side electrode 13, made of an Al film having a thickness of 100 nm and configured to function as a reflecting film also. The beam 14 is constructed in a bridge form, in which both ends thereof are supported.
In the optical MEMS device 11, the beam 14 is displaced by electrostatic attractive force or electrostatic repulsive force generated between the beam 14 and the substrate side electrode 13 depending on electric potential applied between the substrate side electrode 13 and the actuation side electrode 16. For example, as illustrated by the solid and dotted lines of FIG. 2, the beam 14 is displaced to be parallel to and to be depressed toward the substrate side electrode 3.
The optical MEMS devices 1 and 11 may be used as optical switches that are provided with switch functions in such a way as to radiate light onto the surfaces of actuation side electrodes 4 and 16 which also function as reflecting films and detect reflected light having one direction based on the fact that the reflected directions of light are different depending on the actuated positions of the beams 4 and 14.
Furthermore, the optical MEMS devices 1 and 11 may be used as optical modulation devices for modulating the intensity of light.
When the reflection of light is utilized, the intensity of light is modulated using the amount of reflected light per unit time in one direction by vibrating the beam 4 or 14.
In contrast, when the diffraction of light is utilized, a light modulation device is constructed by parallelly arranging a plurality of beams 6 or 14 with respect to a common substrate side electrode 3 or 13, the heights of actuation side electrodes also functioning as light reflecting films are changed by the approach and separation of alternate beams 6 or 14 to and from the common substrate side electrode 3 or 13, and the intensity of light reflected by the actuation side electrodes is modulated by the diffraction of light. This type of light modulation device is a so-called spatial modulation device.
FIG. 3 illustrates the construction of a Grating Light Valve (GLV) device that was developed as a light intensity conversion device for a laser display, that is, a light modulator.
The GLV device 21, as illustrated in FIG. 3, is constructed in such a way that a shared substrate side electrode 23 is formed on an insulated substrate 22, such as a glass substrate, and a plurality of beams 24, in the present embodiment, six beams 24 (241, 242, 243, 244, 245 and 246), are arranged parallel to each other across the substrate side electrode 23 in a bridge form. The construction of the substrate side electrode 23 and the beam 24 is the same as that described above in conjunction with FIG. 2.
The beams 24, which include bridge members 25, and actuation side electrodes 26 configured to be disposed on the bridge members 25 and also to function as reflecting films, are commonly called “ribbons”.
When a small amount of voltage is applied between the substrate side electrode 23 and the actuation side electrodes 26 also functioning as reflecting films, the beams 24 move toward the substrate side electrode 23 due to the above-described electrostatic phenomenon. In contrast, when the application of the voltage is stopped, the beams 24 are separated from the substrate side electrode 23 and return to the initial positions thereof.
In the GLV device 21, the heights of the actuation side electrodes 26 are alternately changed by an operation in which the plurality of beams 24 approach or are separated from the substrate side electrode 23 (that is, the approach or separation of the plurality of beams 24) and the intensity of light reflected by the actuation side electrodes 26 is modulated by the diffraction of light (a single light spot is radiated onto a total of six beams 24).
Meanwhile, the above-described diffractive light modulator is a device that changes optical signals by actuating an upper micromirror layer. In this case, notwithstanding that the upper micromirror layer must be maintained at an initial position after actuation, the upper micromirror layer may be located at a position other than the initial position thereof (drift) due to the environment and the elapse of time. When the position of the upper micromirror layer is restored to the initial position, the performance of the diffractive light modulator can be maintained.